Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
FIG. 1 schematically illustrates a vehicle powertrain 10. The flow of mechanical power is indicated by thick solid lines whereas dotted lines indicate the flow of control signals. Power is provided by engine 12. Transmission 14 adjusts the speed and torque of the power to suit vehicle needs. Differential 16 divides the power between left and right drive wheels 18 and 20 while permitting slight speed differences as the vehicle turns a corner. Some of the engine power is diverted by front-end accessory drive 22 to drive accessories that are not directly related to propulsion. For example, power may be provided to an alternator 24 to generate electrical power and to an air conditioning compressor 26 to cool the passenger cabin. The engine torque diverted to an accessory such as an alternator or an air conditioning compressor may be called an accessory drive torque. Controller 28 sends signals to the engine, transmission, and the accessories to coordinate their operation. Controller 28 may be a single microprocessor or may be multiple communicating micro-processors.
FIG. 2 schematically illustrates a Dual Clutch Transmission (DCT) 14. Input 30 is adapted for coupling the crankshaft of engine 12, potentially via a damper assembly that reduces the transmission of engine pulsations. Ring gear 32 is fixedly coupled to differential 16. First output pinion 34 is fixedly coupled to first layshaft 36 and meshes with ring gear 32. Second output pinion 38 is fixedly coupled to second layshaft 40 and also meshes with ring gear 32. First friction clutch 42 selectively couples input 30 to solid shaft 44, while second friction clutch 46 selectively couples input 30 to hollow shaft 48 which is concentric with solid shaft 44.
Gears 50 and 52 are supported for rotation about first layshaft 36 and mesh with gears 54 and 56 respectively which are fixedly coupled to solid shaft 44. Coupler 58 selectively couples gear 50 or 52 to first layshaft 36. Gear 60 is supported for rotation about second layshaft 40 and meshes with gear 62 which is fixedly coupled to solid shaft 44. Coupler 68 selectively couples gear 60 to second layshaft 40. When couplers 58 or 68 have coupled one of gears 50, 52, or 60 to the respective layshaft, a power flow path is established between solid shaft 44 and ring gear 32. Each of these different power flow paths is associated with a different speed ratio. When clutch 42 is also engaged, a power flow path is established between input 30 and ring gear 32.
Gears 70 and 72 are supported for rotation about second layshaft 40 and mesh with gears 74 and 76 respectively which are fixedly coupled to hollow shaft 48. Coupler 78 selectively couples gear 70 or 72 to second layshaft 40. Gears 80 and 82 are supported for rotation about first layshaft 36 and mesh with gear 76 and 70 respectively. Coupler 84 selectively couples gear 80 or 82 to first layshaft 36. When couplers 78 or 84 have coupled one of gears 70, 72, 80, or 82 to the respective layshaft, a power flow path is established between hollow shaft 48 and ring gear 32. When clutch 46 is also engaged, a power flow path is established between input 30 and ring gear 32. The speed ratios associated with clutch 46 are interleaved with the speed ratios associated with clutch 42 such that clutch 42 is used to establish odd numbered gear ratios and clutch 46 is used to establish even numbered gear ratios and reverse. A shaft that does not continuously rotate with the transmission input or transmission output, such as shafts 36, 40, 44, 48, may be referred to as internal shafts.
When a driver selects Drive with the vehicle stationary, coupler 58 is commanded to couple gear 52 to shaft 36 while clutch 46 is commanded to disengage. To launch the vehicle, clutch 42 is commanded to gradually engage. Similarly, when Reverse is selected with the vehicle stationary, coupler 84 is commanded couple gear 82 to shaft 36. Then, clutch 46 is commanded to gradually engage to launch the vehicle. When cruising in an odd numbered gear, clutch 42 is engaged. To shift to an even numbered gear, clutch 46 is disengaged (if it was not already disengaged), and either coupler 78 or 84 pre-selects the destination power flow path. After the destination gear is pre-selected, clutch 42 is released and clutch 46 is engaged in a coordinated fashion to transfer power between the corresponding power flow paths and adjust the overall speed ratio.
Clutches 42 and 46 may be either dry or wet friction type clutches. One or more friction plates are fixedly coupled to one of the elements while a housing with a pressure plate and a reaction plate is fixedly coupled to the other element. The friction plates are between the pressure plate and the reaction plate. If there is more than one friction plate, they are separated by separator plates that are also fixedly coupled to the housing. When the clutch is fully disengaged, the reaction plate and the pressure plate are spaced apart such that the friction plate can rotate relative to the housing with minimal drag torque. To engage the clutch, an actuator causes a normal force that squeezes the friction plate(s) between the pressure plate and the reaction plate. The torque capacity of the clutch is proportional to the normal force and also proportional to the coefficient of friction. If the elements are rotating at different speeds, the clutch exerts torque on each element equal to the torque capacity in a direction tending to equalize the speeds. If the elements are at the same speed, then the clutch transfers as much torque as is applied up to the torque capacity. If the applied torque exceeds the torque capacity, then the clutch slips creating relative speed.